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Turn-Off Pop (contd.)
it there. All this causes pin 7 to go high, disabling the outputs. This condition persists for many seconds until the rails can no longer power
U605 through the zeners, at which time a very faint, low rumble is heard.
Output Overload
The overload circuit is developed around U604, another slow, quad LM324 op amp. The voltage and current are summed to determine if a safe
operating point exists. The unit is allowed to operate down to 2 ohms total load (4 ohms each side). When an additional 8 ohms is paralleled,
giving 1.6 ohms, after a second or two the unit shuts off indefinitely. This protects the outputs from shorted cabinets and from driving too
many speakers. This condition may also occur while the speaker jacks are moved in and out.
The following circuit works for positive waveforms only. The voltage is obtained from a divider on the output, R784 and R777. Current is
derived from R604 and R605, and is inverted and amplified at U604/1. These signals are now out of phase and subtract from each other at pin
9. When current is too large for a given voltage, pin 9 will tend to go more negative. This is because pin 1 is negative during a positive
waveform excursion, and large loads keep the voltage from going high and demand more current. When pin 9 goes lower than the 2.5 –2.65V
reference on pin 10, the output pin 8 will go high. D766 is now forward-biased and starts to charge up C741, a 10uF cap, through R743, a
220K. When this unsafe condition persists long enough to charge C741 and turn on Q742, U605/3 is momentarily pulled down. This causes
U605/1 to go low, and R801, a 100K, provides hysteresis to keep pin 1 low by providing enough current to keep pin 3 lower than pin 2. The
outputs will now be turned off indefinitely. The unit must be turned off for 3 seconds to allow the system to bleed down before output power
can resume.
Since the overload sensing circuit is not made from precision resistors (current sense resistors are 10%) and is somewhat sensitive, occasionally
a 1K resistor must be paralleled with R777. Also the original boards did not have a spot for R741A, which bleeds C741. Without this
bleederC741 may charge up slowly through excess solder flux around the pins of Q742. Whenever output is inhibited for no apparent reason,
check Q742 and find out which of the diodes feeding its base through R743 is responsible.
Thermal Overload
The final method of output disable is provided by U602, which happens to be a fast LF353 dual op amp, although there is nothing fast about
the circuit. Q807 is situated on top of one of the output transistors. As Q807 heats up with the output, pin 2 will drop from about 2.75V. Pin 1
is low until pin 2 drops below the 2.06V reference on pin 3. Pin 1 will go high and stay high enough for the outputs to be disabled by the same
mechanism using Q742 described above. Once again, the LEDs will remain lit and the preamp will continue to function while the output is
off. During this time the heat sink will continue to cool. To resume power at the instant of shutoff, not only must the power switch be turned
off for 3 seconds, but the outputs must cool off as well. Q782 senses the temperature of the other channel in a stereo unit.
Defeating the Disable Circuit
There will be situations where the auto shutoff feature of the microamplifier will be triggered by the noise from other equipment that is either
connected to the same AC line or is operated in close proximity to the amp. An easy way to prevent this circuit from tripping is described.
This method also disables the thermal trip circuit; however the thermal trip circuit does not fire under most operating conditions.
Procedure: Remove the bottom cover of the amp. Look through the side of the amp with the heat sink down and the front panel on the left.
On the left side of the lower circuit board you will see an MPSA06 transistor labeled Q742. Directly to the right of Q742 and also on the board
edge is capacitor C741. To defeat tripping, solder a wire across C741.
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